Studies of optical pulsations from HZ Herculis/Hercules X-1: a determination of the mass of the neutron star.

In 500 hours of optical observations of this binary system we have repeatedly detected optical pulsations at the 0.1-0.3 percent level. These pulsations are present only for particular well- defined values of the binary and 35-day phases. Positions of the pulsation-emitting regions, projected onto the orbital plane, have been measured, and three distinct regions have been resolved. A simple model is put forth which accounts for the observed binary behavior, which gives a direct determination of the mass ratio, MHZHer/MHerX - 1 = 1.68 + 0.10 and which establishes that the spin of the pulsar is prograde. Additionally, it is shown that Hz Her fills its Roche lobe. Using the above, the known X-ray eclipse duration, and the mass function, we calculate the orbital inclination to be i = 870 + 30 and the masses to be = 1.30 + 0.14 M . and MHZHer = 2.18 + 0.11 M0. Subject headings: stars: binaries - stars: neutron - X-rays: sources